Generally, condenser microphones are parts that convert the sound pressure of externally input acoustic signals into a voltage signal. One such condenser microphone is a digital condenser microphone (Electret Condenser Microphone: ECM) which is characterized in that an existing post-processing stage is remarkably simplified and the noise interference of analog signals is eliminated. However, such a digital condenser microphone must be essentially provided with a preamplifier for amplifying input signals and eliminating noise from the input signals because detection sensitivity to externally input acoustic signals is low. In particular, in recent condenser microphones, as condensers are gradually becoming small in size and integrated, active capacitance (typically, about 1 pF to 10 pF) greatly decreases, and then drop in sensitivity is greater.
FIG. 1 is a circuit diagram showing a conventional digital condenser microphone.
Referring to FIG. 1, the conventional digital condenser microphone includes a microphone sensor unit 1, a high-impedance interface unit 2, a Direct Current (DC) cut-off condenser 3, a preamplifier 4, and an Analog to Digital (A/D) converter 5.
The microphone sensor unit 1 includes an input power source Vs and a condenser Cmic for a microphone, and is configured to detect an acoustic signal Vs from the outside of the microphone and output the detected acoustic signal as a voltage V1.
The high-impedance interface unit 2 includes a resistor R1 having a high-input impedance at the input terminal of a Junction gate Field-Effect Transistor (JFET). The high-impedance interface unit 2 receives the voltage V1 output from the microphone sensor unit 1 and then outputs a voltage V2 via current control.
The DC voltage cut-off condenser 3 cuts off a DC voltage and only an acoustic signal is converted into current by a resistor R2.
The acoustic signal is converted into the current by the resistors R2 and R4 of the preamplifier 4 and is output as a voltage V3 by resistors R3 and R5 in a subsequent stage, and the voltage V3 is input to the A/D converter 5. In this case, the gain of the preamplifier 4 is determined by R3/R2 (R5/R4).
As described above, the conventional preamplifier is configured such that the input voltage V1 undergoes several stages of voltage/current and current/voltage conversion via the impedance interface unit 2 and the input resistors R2 and R4 of the preamplifier 4 and then the amplified voltage signal V3 is output.
However, the conventional digital condenser microphone in which the preamplifier is implemented using the JFET is vulnerable to noise generated by the input power source. Further, disadvantages of the conventional digital condenser microphone are that it requires an individual element, such as the JFET, and that it is inappropriate to integrate the microphone interface, the preamplifier, and the A/D converter into a single chip due to an increase in area attributable to the high-impedance resistor at the input terminal and the DC voltage cut-off condenser.